The skull of Velociraptor - Acta Palaeontologica Polonica

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212 Skull of Velociraptor: BARSBOLD & OSMOLSKA process in Herrerasaurus. In Dromaeosaurus, the caudal margin of the caudodorsal ex- tension of the splenial dorsally contacts the rostral margin of the prearticular, just as it does in Velociraptor (Barsbold 1983: fig. 13a). Contacts of the caudodorsal and caudoventral flanges of the splenial in Deinonychus is erroneously reconstructed by Ostrom (1969b: fig. 16), among others, the Meckelian fenestra not being marked. How- ever, judging from the incised rostral margin of the splenial, identical with those in Drornaeosaurus and Velociraptor, this fenestra had to be present also in Deinonychus. Thus differences with the Velociraptor mandible may be less significant than implied by Ostrom's reconstruction. The angular in Deinonychus is similar, although the contact surface for the splenial and the floor of the Meckelian canal are shallower. In Dromaeo- saurus, the angular is relatively shallower: it forms at most a third of the height of entire labial wall of the mandible. In this respect, it resembles more the angular in some carnosaurs. The most rostral portion of the prearticular is not preserved in Deinonychus. In Dromaeosaurus, the rostral embayment of the prearticular is less deep and the splenial process more stout. The coronoid is very similar in Dromaeosaurus and Velociraptor, although the furrow along the rostral margin seems to be lacking in the for- mer. The coronoid is unknown in Deinonychus. The surangular in Deinonychus is repre- sented by an isolated caudal fragment and it differs from this bone in Velociraptor in the presence of a ridge that projects medially along the dorsal border of the adductor fossa. The marginal portion of the surangular is well exposed medially in ZPAL MgD-1/97 and it is flat, devoid of any ridge. In Dromaeosaurus, the surangular is relatively deeper be- hind the external mandibular fenestra and it forms caudally a larger portion of the ventral margin. The surangular foramen is much smaller in this genus than that in Velociraptor. The articular in Velociraptor seems similar to the articular in Dromaeosaurus. A com- parison of casts of the right surangular (YPM 5234) and the right articular (YPM 5232) of Deinonychus (the two bones may come from the same mandible, because they articu- late perfectly with each other) with our specimens of Velociraptor shows that the articular on Ostrom's figure (1969b: fig. 22) is incorrectly oriented. The 'medial' surface of the bone (fig. 22a) is actually the ventral one, while that determined as lateral (fig. 22b) appears to represent, in fact, the slanting caudal extremity of the bone. The surface indi- cated on this figure as the 'area overlain by the surangular' was located dorsal to the tu- ber labeled by Ostrom as the insertion site of rn. depressor mandibulae; presumably it faced caudodorsally, as in Velociraptor. This surface occurs in many theropods, e.g., in the Tyrannosauridae (Osborn 1912: fig. 4) and Bagaraatan, although it is rather vertical in the tyrannosaurids, and slants slightly in Bagaraatan (Osm6lska 1996: fig. 2e, f). Dentition There are four premaxillary teeth in all our specimens, and they are weakly curved. The two rostralrnost teeth preserved in GIN 100124 are of about equal size and nearly twice as long as the third and fourth teeth (Fig. 3A). The maxilla bears 11 teeth and within the rostral half of the tooth row, every second tooth is longer than the neighbouring teeth. As a result, the tips of the functioning teeth are separated by large spaces. The maxillary teeth are slender and caudally curved. Carinae of these teeth are poorly preserved and it is only possible to note that serrations on the mesial carina are distinctly smaller than those on the distal carina. The denticles on the mesial carina are strongly worn and cannot be measured. Those on the distal carina are better preserved, and there are nine denticles per 2 rnm along the medial section of the carina. The dentary teeth cannot be counted in specimens at our disposal, because mandi- bles are forced under the skull. However, in the holotype the number of dentary teeth is 14 to 15.
ACTA PALAEONTOLOGICA POLONICA (44) (2) 213 In the Djadokhta Fm sediments at Bayn Dzak (= Shabarakh Usu), the type locality of V mongoliensis, numerous shed dromaeosaurid teeth were found that most probably are teeth of this species. All are between 9 and 14 mm (usually 10 mm) long and 3-5 mm (usually 5 rnm) wide across the base of the crown. Some of them (Fig. 2C) have preserved serrations on the mesial carina, but these are most easily visible along the distal portion of the crown; close to the tip, the serrations are worn off. In these teeth, the average number of denticles on the mesial carinae is 7 per 1 mm. The distal carina is thin and serrated, the number of denticles being, on the average, 5 per 1 mm. Denticles are somewhat more crowded close to the base of the crown. Some of these teeth have a wear facet on one side, which is located near the tip of the crown. The interdental plates are indistinguishable in Velociraptor, their presence being probably obscured by fusion, as in other dromaeosaurids. However, the lingual margin of the tooth sockets is as high as the labial margin, which, as suggested by Currie (1995), may indicate that the plates are present. In Deinonychus, there are four premaxillary and 15 maxillary teeth, and a maxi- mum of 16 dentary teeth. The discrepancy between numbers of teeth in the upper and lower jaws is smaller (or, maybe, none) in Velociraptor, 13-15 teeth being present in the upper jaw and 14-15 teeth in the mandible. Sues (1977a) has counted only three premaxillary teeth in the holotype of V mongoliensis. However, on all other specimens there are four premaxillary teeth as in other dromaeosaurids and in most other theropods. According to Sues (1977a) there are nine or ?ten maxillary teeth in the holotype of V mongoliensis. Nine teeth are present in the maxilla of Dromaeosaurus. Discussion Skulls in Deinonychus antirrhopus and Velociraptor mongoliensis, as well as in Saurornithoides langstoni were considered by Paul (1988) as much more similar to each other than either is to the skull in Dromaeosaurus albertensis, and consequently he assumed that Deinonychus and Saurornitholestes are junior synonyms of Velo- ciraptor. This opinion has not been supported by Witmer & Maxwell (1996), who have studied new, more complete material of D. antirrhopus, and have drawn attention to some previously unknown features of this species. For example, they stated that the skull was more robust in D. antirrhopus than in V mongoliensis. These new data on D. antirrhopus, as well as the redescription of Dr. albertensis by Currie (1995), have ren- dered invalid some evidence quoted by Paul in favour of his hypothesis, whereas others appear to be dromaeosaurid synapomorphies. For example, the nasal is not 'de- pressed' in Deinonychus (it is unknown in Dromaeosaurus and other dromaeosaurids), the maxillary alveolar border is also slightly convex in Dromaeosaurus (Currie 1995: figs 1,2), the cross-section of the lacrimal shaft is also U-shaped in Utahraptor (and in some other theropods; the shaft is not preserved in Dromaeosaurus; the lacrimal is unknown in Saurornitholestes), the quadratojugal also has the inverted T shape in Dromaeosaurus, and, as deduced by Currie (1995), the frontal process of the post- orbital was also upturned in Dromaeosaurus. Our present study has farther increased the number of differences between V mongoliensis and D. antirrhopus. In the latter species, the maxilla is more robust, the antorbital fenestra is longer and distinctly triangular, the supratemporal fenestra is nar- rower but longer, and is bounded laterally by the straight supratemporal arcade. In V mongoliensis, the supratemporal arcade is laterally bowed and the fenestra is sub-
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The skull of Velociraptor - Acta Palaeontologica Polonica

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